1. Field of the Invention
The present invention is directed towards a process for the production or coating of granules in a fluidized bed, in which a liquid medium containing a granule- or coating-forming compound or at least one reaction component therefore is applied by spraying particles located in the fluidized bed and having a smaller mean diameter than that of the particles to be produced, and at the same time solvent contained in the medium is vaporized. The process is directed in particular towards the production of organic or inorganic granules, as well as towards the coating of granules with an organic or inorganic coating material. The present invention is also directed towards an apparatus for performing the process according to the present invention, including a fluidized-bed spray granulation installation with which organic solutions or suspensions can be handled safely. The present invention also provides granules obtainable by the process having a substantially spherical habit, and coated granules. The granules or the coating include one or more organic or inorganic compounds.
2. Discussion of the Background
Ever higher demands are being made by the markets on chemical products in the form of solids in respect of their bulk properties. For example, the products that are produced should be dust-free and readily pourable, as well as having a narrow particle size distribution and as high a bulk density as possible.
Although virtually spherical and hence readily pourable particles can be produced by known spray-drying processes, such particles are hollow spheres having a low density and an undesirable tendency to form dust. Fluidized-bed spray granulation, on the other hand, permits the production of approximately spherical solid particles.
An overview of known processes and apparatuses for fluidized-bed spray granulation is given by Hans Uhlemann, Chem.-Ing.-Tech.62 (1990), pages 822-834. Fundamental features of fluidized-bed spray granulation are the formation of a stable fluidized bed inside a granulator (=reactor), the application of the liquid medium, which contains granule-forming material in the form of a solution, suspension or melt to the particles of the fluidized bed by spraying, and the vaporization of the solvent contained in the liquid medium, which takes place at the same time. During fluidized-bed spray granulation, the particles grow and particles of the desired target grain size are separated from the fluidized bed in a suitable manner. Fine particles separated off with the fluidized-bed gas are recycled into the process in a suitable manner. Uhlemann teaches different process variants, measures for injecting a liquid medium into the fluidized bed, for removing dust from the outgoing air and for controlling the moisture content and size of the granules. In all the embodiments of Uhlemann, heated air is always used as the fluidized-bed gas, which serves the purpose of fluidisation and at the same time is the energy carrier. There is no indication in the Uhlemann document that the liquid medium to be sprayed into the fluidized-bed reactor is an organic solution, or as to how resulting safety problems can be overcome.
In the fields of application for fluidized-bed spray granulation mentioned by Uhlemann, aqueous solutions or suspensions are evidently always used as the medium to be sprayed into the reactor.
Chemische Produktion 6/92, pages 18-21 demonstrates the principle of action of a continuous fluidized-bed granulation drier which is suitable, as well as for drying, for the agglomeration of pulverulent substances, for the coating of disperse granular substances, as well as for carrying out chemical reactions between solid and fluid phases. In general, hot air, but also hot gas, is fed to the drier as the drying medium via a specially designed in-flow base. To intensify the transfer of heat, during circulating-air operation a portion of the outgoing air can be fed back to the heat exchanger as circulating air and is available to the reactor again as the drying medium. According to an alternative embodiment, the outgoing air leaving a fluidized-bed spray granulator is used to pre-heat fresh air used as the drying medium. This document also makes no reference to the use of organic solutions as the liquid medium to be sprayed, nor does it give any indication as to which other hot gas should be used instead of hot air and how the risk of the formation of explosive solvent vapours can be avoided without too great a technical outlay.
According to DE patent specification 197 04 180, alkali cyanide granules are produced from a HCN-containing gas and an aqueous solution of an alkali hydroxide in a fluidized bed. Instead of air, with which HCN explosive mixtures would form, nitrogen or superheated steam is used as the fluidizing gas.
To the knowledge of the inventors of the present invention, no industrial process is as yet known according to which the fluidized-bed spray granulation process has been used to produce bulk materials that have been produced using solutions or suspensions in an organic solvent that forms explosive vapours with air. The use of organic solutions is therefore a hurdle in terms of process technology and safety technology when performing a fluidized-bed spray granulation process. To avoid the formation of explosive vapours of air with the organic solvent, a person skilled in the art could use nitrogen instead of air as the fluidized-bed gas in the generic process. Such a procedure would not be very economical, however, because of the expensive fluidized-bed gas and additional necessary precautions for recovering the organic solvent from the waste gas.